The present invention relates to a photoelectric conversion apparatus which includes a plurality of linearly arranged photoelectric conversion elements and is used, for example, as a linear image sensor.
In an optical character reading apparatus or a facsimile apparatus, the linear image sensor used receives reflected light from an object to be read out and converts the incident light into electric signals. The linear image sensor includes a photoelectric conversion element array of a number of linearly arranged photoelectric conversion elements, and a circuit for reading out electric image signals from the respective photoelectric conversion elements. The conventional photoelectric conversion apparatus shown in FIG. 1 includes a photoelectric conversion element array of a number of linearly arranged photoelectric conversion elements, e.g., photodiodes 1.sub.1 to 1.sub.n ; and a circuit device for sequentially reading out the electric image signals from the respective photodiodes. The circuit device includes a common output line 4; analog switches 3.sub.1 to 3.sub.n which comprise, e.g., MOSFETs, and are respectively connected between the common output line 4 and the photodiodes 1.sub.1 to 1.sub.n ; and a pulse generating circuit 2 for generating scanning pulses .phi..sub.1 to .phi..sub.n and an inhibit pulse .phi..sub.IN for use in controlling the ON/OFF operation of the analog switches 3.sub.1 to 3.sub.n. The sensor further includes a signal processing circuit which reads out the signals from the photodiodes 1.sub.1 to 1.sub.n onto the common output line 4, and then produces the signals as a video signal 7. The signal processing circuit includes a capacitor C.sub.V connected to the common output line 4 and ground; an analog switch 5 connected between the common output line 4 and ground, the ON/OFF operation which is controlled by a clear pulse .phi..sub.R from the pulse generating circuit 2; and an operational amplifier 6 connected to the common output line 4. If it is assumed that only the photodiodes 1.sub.1 to 1.sub.5 are arranged in the apparatus shown in FIG. 1, the timings of the pulses generated by the pulse generating circuit 2 and the timings of the signals read out on the common output line 4 hold the relationship shown in FIG. 2. Referring to FIG. 2, the scanning pulses .phi..sub.1 to .phi..sub.5 have a pulse width T and sequentially and repeatedly scan the analog switches 3.sub.1 to 3.sub.5 (FIGS. 2A to 2E). When the time period for completing one scanning cycle in accordance with scanning pulses .phi..sub.1 to .phi..sub.5 is defined as a one line scanning time period, each diode stores a charge in response to the project light intensity during this one line scanning time period. For example, to prevent a simultaneous turn-on of analog switches 3.sub.1 and 3.sub.2 when scanning pulse .phi..sub.1 is switched to scanning pulse .phi..sub.2, an inhibit pulse .phi..sub.IN (FIG. 2F) which is synchronous with the switching timing from pulse .phi..sub.1 to pulse .phi..sub.2 is applied to the analog switches 3.sub.1 to 3.sub.5. In the time period in which the inhibit pulse is applied, the analog switches 3.sub.1 to 3.sub.5 are turned off. When T1 is the rising time of the scanning pulse .phi..sub.1, T5 is the falling time thereof, T2 is the falling time of the inhibit pulse .phi..sub.IN at time T1, and T4 is the rising time of the inhibit pulse .phi..sub.IN corresponding to time T5, the analog switch 3.sub.1 is turned on from the time T2 to T4. Then, charge from the photodiode 1.sub.1 is read out onto the common output line 4 from the time T2 to T4. The charge is charged on the capacitor C.sub.V connected to the common output line 4. The charge time constant t of the capacitor C.sub.V is about 0.7C.sub.V .multidot.r where r is the ON resistance of the analog switch 3.sub.1. Since the clear pulse .phi..sub.R rises at the time T3 and the analog switch 3.sub.5 is turned on, the charge on the capacitor C.sub.V is discharged in the time interval between the times T3 and T5. Then, the operational amplifier 6 produces a video signal 7, i.e., the video signal e.sub.S1 shown in FIG. 2I, from time T2 to T3. The pulse generating circuit 2 may comprise a shift register, or a counter which receives a start pulse 8 and clock pulses 9. For the sake of simplicity, a description has been made only with reference to pulse .phi..sub.1. However, the mode of operation for the other scanning pulses .phi..sub.2 to .phi..sub.5 remains the same.
The defects of the conventional photoelectric conversion apparatus shown in FIG. 1 will now be described, based on the assumption that the photodiode array only has photodiodes 1.sub.1 to 1.sub.5. The common output line 4 receives the noise signal components as well as the image signal components based on charges stored on the photodiodes. The noise signal components are generated due to the turning on and turning off of the analog switches and gate capacitances of the MOS transistors constituting the analog switches. Accordingly, the noise signals differ for the respective analog switches. Thus, even if the photodiodes 1.sub.1 to 1.sub.5 are not irradiated with light, that is, even in the dark condition, as long as the analog switches are switched, the operational amplifier 6 produces noise signals e.sub.N1 to e.sub.N5, as shown in FIG. 2H. The noise signals have the varied amplitudes shown in FIG. 2H, for the reason given above.
The case wherein uniform light is projected onto the photodiodes 1.sub.1 to 1.sub.5, i.e., wherein bright lighting conditions are present, may be described as follows. Under such conditions, the amplitudes of the image signals read out from the photodiodes 1.sub.1 to 1.sub.5 must be the same. However, since the corresponding noise signals are added to these image signals, the operational amplifier 6 produces the video signals e.sub.S1 to e.sub.S5 shown in FIG. 2I. The ON resistances of the analog switches 3.sub.1 to 3.sub.5 change in accordance with the ambient temperature. Accordingly, variations in the video signals are further enhanced. When such noise signals e.sub.N1 to e.sub.N5 are generated, the photoelectric reading of, e.g., slips, becomes unreliable, and the processing of slips becomes impossible.